I use BBEdit for modifying plist files in /Library/Preferences, which has the advantage of handling the authorization needed to obtain write permission, and then save the files with their original attributes.
As explained in @klanomath's answer, disable SIP, but make sure you mount /Volumes/Macintosh HD with read write options inside the recovery mode:
mount -uw /Volumes/Macintosh\ HD
You can then continue with the remaining steps to edit the file.
Then generate and tag new APFS System Snapshot as follows:
You can't change what the Context menu item does in so far that it is fixed to look up a word in the macOS dictionary.
You can't even change it to use a specific dictionary.
You can, however, make some changes. The look up that is done is according to the preferences of the Dictionary app and it is possible to change the dictionary used to another that Apple ...